Add a summary to the end of successful runs for everything using the new looping base command, similar to what we do for summarizing failures. This will make it easy to manually check results for PRs that we know should be changing the set of run packages (adding a new package, adding a new test type to a package, adding a new test type to the tool), as well as spot-checking when we see unexpected results (e.g., looking back and why a PR didn't fail CI when we discover that it should have).
To support better surfacing skips, this restructures the return value of `runForPackage` to have "skip" as one of the options. As a result of it being a return value, packages that used `printSkip` to indicate that *parts* of the command were being skipped have been changed to no longer do that.
Fixes https://github.com/flutter/flutter/issues/85626
Migrates firebase-test-lab to use the new package-looping base command.
Other changes:
- Extracts several helpers to make the main flow easier to follow
- Removes support for finding and running `*_e2e.dart` files, since we no longer use that file structure for integration tests.
Part of https://github.com/flutter/flutter/issues/83413
